class RbNaCl::HMAC::SHA512256

def initialize(key)

see https://tools.ietf.org/html/rfc2104#section-3

The key for HMAC can be of any length.
RFC 2104 HMAC

Create instance without checking key length 
def initialize(key)
  @key = Util.check_hmac_key(key, "#{self.class} key")
  @state = State.new
  @authenticator = Util.zeros(tag_bytes)
  self.class.auth_hmacsha512256_init(@state, key, key.bytesize)
end